It is proposed to pursue and amplify our studies on the biologic properties of avian myeloblastosis virus, the role of nontransforming avian leukosis viruses in rescuing AMV from nonproducer leukemic cells, and the envelope properties which determine the host range of the rescued avian leukosis viruses. Studies in progress or planned for the immediate future may be summarized as follows: a) Search for possible mechanisms involved in spontaneous regression of leukemia, including development of methods to test if regression is age and/or virus- dependent or if humoral or cell mediated immune responses are implicated. b) To obtain a transforming AMV free of nontransforming associated viruses and to study its leukemogenic properties. c) If the transforming principle of AMV is maintained or modified by long-term passage of the virus in cells on nonhematopoietic origin. d) To characterize the several factors involved in colony formation of normal and leukemic cells with particular emphasis that this type of study may have in cell growth regulation.